1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to devices and systems used in marine exploration and, more particularly, to deflectors for controlling seismic source and/or seismic receivers positioning.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which is especially helpful in determining the above-noted reservoirs. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections and/or refractions to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating seismic waves and recording their reflections off geological structures present in the subsurface is now illustrated in FIG. 1. System 100 includes a vessel 102 that tows two source arrays 110a and 110b (it is also possible to tow only one source or more than two sources but, for simplicity, the novel features are discussed with regard to two source arrays) and plural streamers 120. Streamers 120 are connected to the vessel through lead-ins 122, while source arrays 110a and 110b are connected to the vessel through dedicated cables 112. Each source array 110a or 110b may include sub-arrays 114, each sub-array having plural individual source elements 116. Deflectors 140 are provided on the sides of this arrangement to maintain a transverse distance (relative to the path of the vessel) between streamers 120. Deflectors 140 are connected to vessel 100 via wide tow cables 142, and spread ropes or cables 144 are used to separate the streamers from each other. Note that the terms “rope,” “cable” and “wire” are used sometimes interchangeably in this document. Thus, these terms should not be construed in a narrow sense, but rather as those skilled in the art would expect. The number of streamers or individual source elements is exemplary and not intended to limit the applicability of the novel concepts.
A single sub-array 114 is shown in FIG. 2. Sub-array 114 includes one or more floats 160 from which individual source elements 116 are suspended with cables, chains or ropes 162. In one application, clusters of individual source elements are provided at location 116. Various cables and hoses connect individual source elements 116 to the vessel for providing electric power, compressed air, data transmission, etc. For example, a hose 164 provides compressed air and a cable 166 provides electric power and/or data transmission.
Source bases 118 are connected to each other via links 170 and also to a bell housing 180 via a connection 182. In one application, links 170, bell housing 180 and connection 182 may form an enclosure in which the various cables 164 and 166 are entering. Bell housing 180 may be made of a resistant material, for example, stainless steel. A bend restrictor device 190 may be connected to the bell housing 180 and also to vessel 100 via an umbilical 192. Bend restrictor device 190 is configured to prevent an over-bending of the front part of the source array due to the towing force applied via umbilical 192. Bend restrictor device 190 may also be made of a resistant material. In one application, bell housing 180 may be directly connected to umbilical 192.
Returning to FIG. 1, to prevent the sub-arrays from moving along a cross-line direction Y, various cables 170 and 180 may be used to anchor the sub-arrays to lead-ins 122. However, these cables are not enough for maintaining equal separation of the sub-arrays from each other and also from the lead-ins. FIG. 3 shows a situation in which cables 170 and 180 are slack because of various underwater currents (not shown) that move the sub-arrays. For this reason, winches 172 (see FIG. 1) may be added to adjust the length of cables 170 as desired, to avoid the slacking.
However, even with such winches, if the water currents are strong and/or non-symmetrical, various sub-array separation problems may still appear as illustrated in FIG. 4. The configuration of FIG. 4 shows water currents 192 acting on vessel 102, which changes its orientation relative to the intended path 190, and also acting on sub-arrays 110a and 110b. Hydrodynamic forces 194 on the port side of the source may be stronger than the same forces 196 on the starboard side. For this reason and because of the misalignment of the sub-arrays, the separation between sub-arrays starts to deviate from preset values, i.e., separation 198 on port becomes too wide and separation 199 on starboard becomes to small (the array 110b collapses). This alteration of source array geometry is reflected in the recorded traces and degrades the overall quality of the image of the surveyed subsurface. Deflectors 182 may be attached to the umbilical of each sub-array to stabilize the sources, but the existing deflectors have their own limitations (e.g., instability) and are only partially successful in reaching this goal.
Thus, there is a need to develop a better deflector for preventing the above-noted problems.